Add function to correlate expression with gene program scores

[jr-leary7]

• main goal is to find genes that are "important" to each identified gene cluster / program
• do this by computing Spearman correlation of gene expression with per-cell module scores from geneProgramScoring() & getting associated p-values
• this is kind of inspired by what CellRank does to identify lineage drivers (source)
• this is a future enhancement & won't be worked on until after the preprint is submitted (I promise!)